The invention concerns a process for track jumping for a track control circuit with an optical scanning device, at which a first and a second light beam are reflected by a information medium to one photo diode each and at which, from the output signal of the two photo detectors, a sinus-shaped track error signal, that is the deviation of actual value from nominal value, are produced by subtraction.
CD-players, video record players, DRAW-disc-player or magneto-optical recording- and reproducing-sets are for instance equipped with an optical scanning device.
Construction and function of an optical scanning device, a so-called optical pick-up, are described in Electronic Components & Applications, Vol. 6, No. 4, 1984 on the pages 209 to 215.
The lightbeam emitted by a laser diode is focused by lenses on the CD-record and from there reflected to a photo detector. The data stored on the CD-record are obtained from the output signal of the photo detector, as is the actual value for the focus- and for the track-control circuit. In the literature reference the deviation of the actual value (quantity) from the nominal value (quantity) for the focus control circuit is named as focusing error, while for the deviation of the actual value from the nominal value of the track control circuit, the expression radial tracking error is chosen.
A coil serves as positioning element for the focus control circuit. By the magnetic field of the same, a lens is movable along the optical axis. The focus control circuit causes now, by shifting of the lens, that the light beam emitted by the laser diode will always be focused on the CD-record. With a track control circuit, often called also radial drive, the optical scanning device is movable relative to the CD-record in radial direction. By this the light beam can be guided on the spiral-shaped data track of the CD-record.
In some sets the radial drive is constructed using a so-called coarse- and a so-called fine-drive. The coarse-drive is for instance realized as a spindle, by which the total optical scanning device, the laser diode, the lenses, the prism beam splitter and the photo detector, are radially movable. With the fine-drive the light beam is additionally movable in radial direction or e.g. tiltable about a predetermined small angle. With the fine-drive, the light-beam can be moved a small path--about 1 mm--along a radius of the CD-record.
In order to achieve an unobjectionable reproduction of the data, be it e.g. picture and audio at a video record player or only audio at a CD-record or the data at a magneto-optical record, then it is necessary to focus the light beam exactly on the record and also guide precisely along the data track of the record.
FIG. 1 shows the photodetector PD of the optical scanning device of a CD-player, in which three laser beams L1, L2 and L3 are focused on the CD-record. The laser beam L2 and L3 are the diffraction beams of +1 and -1 order. Such a scanning device is named a Three-Beam-Pick-Up in the literature reference cited in the beginning, because this device operates with three light beams.
At the photodetector, four square-shaped photodiodes A, B, C and D are jointly arranged, so that they form a square. Ahead of the square formed by the four diodes A, B, C and D is a rectangular photodiode E; behind the square is a further photodiode F. The mean laser beam L1, which is focused on the four photodiodes A, B, C and D, produces the data signal HF=AS+BS+CS+DS and the focus error signal FE=(AS-CS)-(BS+DS). The two outer light beams L2 and L3, of which the one ahead, L3, is directed to the photodiode E, the one behind, L2, to the photodiode F, produce the track error signal TE=ES-FS. The photo voltages of the diodes A, B, C, D, E and F are respectively labeled with AS, BS, CS, DS, ES and FS.
In FIG. 1 the means laser beam L1 follows exactly the middle of track S. The track error signal TE has zero value. EQU TE=ES-FS=0.
If the means light beam deviates from the middle of a track S, the one diffraction beam moves more to the center of track, while the other diffraction beam radiates on the space between two tracks S. Because the reflection quality of a track differs from the space between tracks, one of the diffraction beams is more strongly reflected then the other one.
FIG. 2 shows the case in which the laser beams L1, L2 and L3 are shifted to the right from the track S. The track error signal assumes a negative value: EQU TE=ES-FS&lt;0.
The positioning element of the track control circuit moves the optical scanning device now so far to left, until the track error signal TE becomes zero.
In the opposite case, when the light beams are shifted to the left from the track, the track error signal will be positive: TE=ES-FS&gt;0. Now the positioning element of the track control circuit moves the optical scanning device so for to right, until the track error signal TE becomes zero. This case is shown in FIG. 3.
If the light beam L1 and the respective diffraction beams L2 and L3 cross several data tracks, the track error signal TE assumes the sine shape shown in FIG. 4.
From the JP-OS 60 10 429 patent a track control circuit is known in which the upper and lower envelope of the high-frequency signal can be recognized, if the light beam crosses data tracks. When the light beam crosses several data tracks, the high-frequency signal collapses regularly between two tracks.
In order to detect the tracks crossed by the light-beam, the envelope of the high-frequency signal will be formed and converted into a rectangular signal, which is supplied to the counting input of a bi-directional counter. In this way the high-frequency glitches are counted by the bi-directional counter.
In order to determine, in which direction the light beam has been moved, radially toward the center or off the center, a so-called direction detecting logic is required. It is known for instance, to supply the track error signal to the D-input of a D-flip-flop and the envelope of the high-frequency signal to the clock input of the flip-flop. The D-flip-flop receives therefore each time a pulse at its clock-input, when the light beam crosses a data track. Because the polarity sign of the track error signal TE depends on the direction, however, in which the light beam leaves a data track, the D-flip-flop is set- its output will be "High"-, in one direction whereas in the other direction its output remains at the opposite at "Low". The signal at the Q-output of the D-flip-flop can therefore serve to determine the direction of counting-forward or backward- of a bi-directional counter. Therefore the bi-directional counter counts, in the case of one direction, forward, while its counts, in the case of the other direction, backward.
These known track control circuits have besides a high effort complexity due to the direction detecting logic, the further drawback, that the control loop of the track control circuit has to be opened for track jumping. The optical scanning device moves therefore without checking by the controller across the tracks to be jumped over on the information medium.